Superheterodyne radio receivers achieve good sensitivity and selectivity by mixing the signal received by the antenna (which has a frequency fRF) with a local oscillator (which has a frequency fLO). What is meant by mixing is that the two are multiplied together. Mathematically, this results in the production of signals at additional frequencies: for example at the sum of the two frequencies and at the difference between the two frequencies. It is the signal at the difference of the two frequencies fIF which is important as it is a much lower frequency and thus can be filtered more easily. The problem with this method is that there are two frequencies at the antenna which produce the same difference frequency: one is higher in frequency than fLO and one is lower in frequency than fLO. Since only one of these is desired to be received, the other (called the image) must be filtered out prior to the mixing operation. It is convenient for the difference frequency fIF to be as low as possible. For this reason, it has become common to use SAW filters to filter out the image because of their sharp filtering properties. However, the ultimate attenuation which SAW filters provide is usually not enough and other filtering must be added to supplement them, which forces fIF to be higher than desirable. One possible solution is to use multiple stages of SAW filtering (and amplification to overcome the loss in the SAW filter) but this increases size and cost and degrades the intermodulation rejection properties of the receiver. Another possible solution is to use an image reject mixer, but this also increases size (unless it is integrated into a chip), cost and current consumption.